The present invention relates to an organic nonlinear optical material and an optical nonlinear device using the same and, more particularly, to a polymer material having a large second-order or third-order optical nonlinear effect and an optical nonlinear device using the same.
Recently, a technique of manufacturing an optical nonlinear device such as an optical modulator or an optical bistable device by using a transparent polymer containing a dye material having a large second-order or third-order optical nonlinearity has been studied. A material used in the technique is generally formed by chemically bonding a dye having a large second-order or third-order nonlinear molecular susceptibility to the main chain of a transparent polymer. An example of the dye material is a conjugate .pi. electron system having a donor or an acceptor. More specifically, it is reported that a second-order or third-order optical nonlinearity was measured by bonding an azo dye (K. D. Singer et al., Appl. Phys. Lett. 53, 1800, (1988)) or a stilbene dye compound R is the copolymer of p-hydroxystyrene and methylmethacrylate to the main chain of polymethylmethacrylate. It is also known that an optical nonlinear coefficient can be increased by increasing the content of an optical nonlinear dye. As a device forming method, there is proposed a method of forming a waveguide optical nonlinear device using a polymer film as an optical waveguide by ion or plasma etching or radiation of visible or ultraviolet rays. When the polymer material is used as a third-order optical nonlinear material, no special treatment need be performed after device formation. However, when the material is used as a second-order optical nonlinear material, poling alignment must be performed to uniformize directions of dipoles in an optical nonlinear dye portion. For this purpose, a poling treatment is performed in a DC electric field.
A waveguide optical nonlinear device for use in optical communications or optical information processing systems is practically designed to be connected to a single-mode optical fiber (P. Kaczmarski et al., IEEE PROCEEDINGS, Vol. 136, No. 3, 1989, pp. 152-158), and a waveguide itself is required to function as a single-mode waveguide (R. Lytel et al., SPIE VOL. 1216, 1990, pp. 30-40). The above references disclose that the thickness of an optical nonlinear waveguide must be 3 to 5 .mu.m or more for this purpose. However, in the manufacture of a waveguide optical nonlinear device, no polymer film satisfactory as a practical optical waveguide can be obtained at a high dye content at which a large second-order or third-order optical nonlinear effect can be obtained. Actually, it is impossible to manufacture an optical nonlinear device which can be incorporated in an optical communication system. More specifically, it is difficult to manufacture an optically uniform film having a uniform thickness corresponding to an optical waveguide size. As a result, only an optical waveguide having a large transmission loss can be manufactured.
A polymer film containing a second-order or third-order optical nonlinear component at a high content and satisfactory as a practical optical waveguide cannot be manufactured as described above mainly for the following reasons. That is, since the polymer film is very hard to dissolve in a polymer solvent, it is impossible to perform manufacture of an optical waveguide film according to a spin coating method or a dipping method which is the most general method in the manufacture of a polymer film in a single step. As a result, the spin coating or dipping method must be repeated a plurality of times to obtain a film thickness required for an optical waveguide. Therefore, a film becomes optically nonuniform on film boundary surfaces formed in the respective steps, and a dimensional precision of a film thickness is very poor.
As described above, a polymer material containing a second-order or third-order nonlinear component at a high content cannot be used as a practical optical nonlinear device material regardless of its large second-order or third-order optical nonlinear coefficient.